Intravascular ultrasound (IVUS) is a medical imaging technology. It uses a specially designed catheter that includes an ultrasound transducer. In the typical application, the catheter is inserted into the vascular system of a patient and moved to an artery or vein of interest. It allows the doctor to obtain an image of the inner walls of the blood vessels, even through intervening blood. Specifically, it allows visualization of the endothelium (inner wall) of blood vessels, and structures within the vessels walls.
In its typical application, IVUS is used in coronary arteries of the heart to locate, identify and characterize atherosclerotic plaques in patients. It can be used both to determine the plaque volume in the blood vessel wall and also the degree of stenosis (narrowing) of the blood vessels. In this way, IVUS is an important technology for the structural analysis of blood vessels.
Optical coherence tomography (OCT) is an emerging technology that also provides structural information similar to IVUS. OCT also uses a catheter that is moved through the blood vessels to regions of interest. An optical signal is emitted from the catheter head and the returning signal is analyzed for phase or coherence in a Michelson interferometer, usually.
OCT has potential advantages over IVUS. Generally, OCT provides the opportunity for much higher spatial resolution, but the optical signals have limited penetration through blood and attenuate very quickly when propagating through the walls of the blood vessels.
An objective to using systems based on OCT and IVUS structural imaging technologies is the early identification of vulnerable plaques since disruption or rupture of atherosclerotic plaques appears to be the major cause of heart attacks and strokes. After the plaques rupture, local obstructive thromboses form within the blood vessels. Both venous and arterial thrombosis can occur. A coronary thrombus often initially forms at the site of rupture of a vulnerable plaque; i.e. at the location of a plaque with a lipid-rich core and a thin fibrous cap (thin-cap fibroatheroma or TCFA).
Another class of intravascular analysis systems directed to the diagnosis and analysis of atherosclerosis uses chemical analysis modalities. These approaches generally rely on optical analysis including near infrared (NIR), Raman, and fluorescence spectral analysis.
Probably the most common and well developed of these chemical analysis modalities is NIR analysis of the blood vessel walls. Similar to OCT, NIR analysis utilizes an intravascular optical catheter. In a typical application, the catheter is driven by a pullback and rotation unit that simultaneously rotates the catheter head around its longitudinal axis while withdrawing the catheter head through the region of the blood vessel of interest.
During this pullback operation, the spectral response of the inner vessel walls is acquired in a raster scan operation. This provides a spatially-resolved spectroscopic analysis of the region of interest. The strategy is that by determining the spectroscopic response of blood vessel walls, the chemical constituents of those blood vessel walls can be determined by application of chemometric analysis for example. In this way, potentially vulnerable plaques are identified so that, for example, stents can be deployed in order reduce the risk of myocardial infarction.
In Raman spectral analysis, the inner walls of the blood vessel are illuminated by a narrow band, such as laser, signal. The Raman spectral response is then detected. This response is generated by the inelastic collisions betweens photons and the chemical constituents in the blood vessel walls. This similarly produces chemical information for the vessel walls.
Problems associated with Raman analysis are, however, that the Raman process is a very weak and requires the use of high power optical signals in order to generate an adequate Raman response. Fluorescence has some advantages in that the fluorescence response is sometimes much larger than the Raman response. Generally, however, fluorescence analysis does not yield as much information as Raman or NIR analysis.
Another advantage of NIR analysis is that the blood flow does not necessarily have to be occluded during the analysis. The judicious selection of the wavelengths of the optical signals allows adequate penetration through intervening blood to the vessels walls and back to the catheter head.
In an effort to obtain the valuable information from both the chemical and structural analysis modalities, hybrid IVUS/optical catheters have been proposed. For example, in U.S. Pat. No. 6,949,072, a “device for vulnerable plaque detection” is disclosed. Specifically, this patent is directed to intravascular probe that includes optical waveguides and ports for the near infrared analysis of the blood vessel walls while simultaneously including an ultrasound transducer in the probe in order to enable IVUS analysis of the blood vessel walls.